1. Field of the Invention
The subject disclosure relates to weather radar systems, and more particularly to an improved system and method for processing weather radar signals.
2. Background of the Related Art
In the past, weather radar processors have been built using the approach of a fixed notch-width infinite impulse-response (IIR) clutter filter followed by time-domain autocorrelation processing (so-called pulse-pair processing). These techniques require minimal storage and very few computational multiply accumulate steps (MAC's) per pulse per range bin. The algorithms are well suited for real time implementation and indeed, since there was no ability to buffer large numbers of T and Q samples, there was little choice.
Despite the advantages of these systems, several major drawbacks exist. For example, the impulse response of the IIR filter is, as the name implies, infinite. Thus, perturbations that are encountered, such as a very large point clutter target or change in the pulse repetition frequency (PRF), effect the filter output for many pulses sometimes effecting the output for several beamwidths. The use of clearing pulses or filter initialization can mitigate the effect of this at the expense of effectively reducing the number of pulses.
Further, the filter width that is necessary to remove clutter bias depends on tile strength of the clutter. If the clutter is very strong, then a wider filter is required since the clutter power will exceed the noise power for a greater fraction of the Nyquist interval. In other words, the fixed notch-width is guaranteed to be either not aggressive enough for strong clutter and overly aggressive in removing weather echoes even when there is no clutter. When no clutter is present, the filter will bias the intensity and velocity estimates when the weather target is in the stop band of the filter (overlapped). Operators are required to manually select a filter that is sufficiently wide to remove the clutter without being too wide that the filter attenuates weather data. Clutter filter maps have been used in some systems to try and address this, but the problem of scan rates and local target properties (i.e., width) change from day-to-day without the easy ability to accommodate this variation
Some other systems have employed fast Fourier transform (FFT) processing. The advantage of an FFT approach is that the ground clutter filtering is made adaptive by searching in the frequency domain to determine the boundary between the system noise level and the ground clutter. The FFT is inherently a finite impulse response (FIR) block processing approach that does not have the transient behavior problems of the IIR filter. It is possible to interpolate over spectrum components that are removed to minimize the effects of filter bias. However, the FFT approach has two distinct disadvantages:
i) the spectrum resolution is limited by the number of points in the FFT which is constrained to be a power of 2 (e.g., 16, 32, 64, 128). Operational systems typically use 32 or 64-point FFT's. If the number of points is low, then clutter will be spread over a larger fraction of the Nyquist domain thus obscuring weather targets; and
ii) to provide the best performance, a time-domain window is applied to the IQ values prior to performing the FFT. When the clutter is very weak, even a rectangular window or uniform weighting is adequate for removing clutter. However, when the clutter is strong, then a more aggressive window such as the Hamming or the very aggressive Blackman must be used to isolate weather signals from strong clutter if at all possible. The drawback of these windows is the effective reduction of the number of samples that are processed since the points at the beginning and end of a time series are weighted less than those in the middle. The result is estimates with a higher variance. Lastly, if a fixed notch-width rather than an adaptive width approach is used, then the same types of problems inherent in the IIR filter will exist. As can be seen from the above, prior art systems and methods have limited processing ability.